IM SET testing - Ham Radio Projects

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IM SET testing

DOUBLE DDS

Hammond case was used for AMPs + LPFs assembling. On the top panel of the case a massive heatsink was mounted , which will eliminate the termal load. From the bottom side of it both amplifiers plus PSU board were fixed.

Assembled IM measurement set  prepared for the first testing.

LEFT switch  :  Double DDS + LPF PSU ON-OFF  (green LED)
                                                       MIDDLE switch  :  AMPs ON-OFF  (red LED).
    Possible measuring modes (one or two channels)  :  DDS only
                                                                                                                                           DDS + LPF
                                                                                                                                           DDS + AMP
                                                                                                                                           DDS + AMP + LPF  


1. DOUBLE DDS ALONE

DDS output level is adjustable from the front panel. See the spectrum analysis from the following pictures. Rohde&Schwarz Spectrum Analyzer FSH6 (100kHz - 6 GHz) was used for this purpose (with wrong date/time preset...). Minimum output level reaches -6.1 dBm, posible maximum is +5.9 dBm . It is 12 dB range of level regulation.


And corresponding oscillograph records of minimum and maximum output level. Good sinusoidal signal is visible for both levels. DDS can be used for IM measuring "as is" thanks to its very high level of OIP3. Its second harmonic is suppressed very good as is visible from the next pictures.

 

2. DDS + LPF

Thanks to a very good linearity of the output signal from DDS there is no necessary to use combination DDS + LPF. Let´s compare the second harmonic levels when signal is going thru LPF from the DDS (just for completeness).


3. DDS + AMP

Amplifier has abt. 35 dB of gain. In practice, there will be necessary to add external attenuators (20 dB resp. 10 dB) in every channel between DDS output and AMP input to have possibility to change continuously output level beginning from 0 dBm to the possible maximum. Connection is possible thru rear panel connectors. 

The following two pictures show that level of the second harmonic in both cases is unacceptable high (20dB attenuator was used at the Spectrum Analyzer input). Next two pictures show oscillograph record of AMP output signals for minimum and maximum signal level incoming from DDS and confirms that amplifiers working outside of linear range of its characteristic. I checked that below +25 dBm the output signal has sinusoidal profile. It will be always necessary to use LPFs along with amplifiers, regardless of signal level. It can be to the benefit of IM features of the entire set.


The following pictures show IM records of the AMP outgoing signals for both levels - the minimum and maximum incoming from DDS (no attenuators between DDS and AMP used). Output from AMPs are IM free for output signals level till abt. +25 dBm (+31 dBm from AMPs). If maximum DDS level will drive AMPs, IM products will be present.


4. DDS + AMP + LPF
Have a look on the following pictures, where output signals from LPFs were recorded....Although amplifiers did not work in a linear range if its characteristics, the output signal from the LPF is sinusoidal....harmonics were supressed...amplifiers were driven directly with minimum and maximum level from DDS.


DDS+AMP+LPF as a set was tested using my Signal Hound USB-SA44B Spectrum Analyzer with software for PC, Agilent Oscilloscope DSO-X-2022, Tectronix Power Supply PWS2323 as a primary PS, step attenuator 0-71dB from M0WWA,  fixed attenuator BIRD 20dB/5W and two home made switchable RF attenuators 0-63dB.

Measurements of whole set I did initially at a minimum level of DDS (abt. -6dBm), the output level from LPF reached  +27.5 to +29.2dBm according to the measured band. Maximum output level for DDS output +6dBm reached from +31.4 to +33.2dBm depending on the used band. See the next table for more detailed results. Current consumption (AMP + LPF) ranged from abt. 800 to 900mA in whole range of DDS level (one channel). Whole set current consumption reaches close to 3 Amps.

BandOutput A min. (+dBm)Output A max. (+dBm)Output B min. (+dBm)Output B max. (+dBm)
16028.032.928.132.9
8027.931.727.931.7
4028.031.528.431.7
3028.331.928.331.9
2028.632.928.833.0
1729.133.229.233.1
1528.531.928.732.1
1228.432.928.533.2
1027.632.227.632.2
627.531.427.731.5

For completeness have a look on second harmonics of pure DDS output signal on each band.


As last result have a look on the two tone test thru whole IM set for 3 diferent levels (min, max, +25dBm).

 
 
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